The present invention relates to a catalyst for selective hydrogenation of diolefinic and styrenic compounds in unsaturated gasolines without hydrogenating the aromatic and mono-olefinic compounds. It can also eliminate marcaptans when they are present in thirst gasolines.
Steam cracking gasolines ale known to contain gum precursors such as diolefins, also styrenic compounds (such as styrene itself), mixed with mono-olefinic and aromatic compounds. Upgrading mono-olefinic compounds involves selective hydrogenation of diolefins and styrenic compounds.
Cracking of hydrocarbons to produce olefins produces a liquid fraction with a portion of the compounds having a boiling point corresponding to traditional gasoline cuts: This mixture contains high quantities of aromatic and mono-olefinic compounds, which endow it with good fuel properties and enable it to be upgraded, either in the xe2x80x9cgasoline poolxe2x80x9d or as a source of aromatic compounds. However, such cracking gasolines contain large quantities of highly reactive compounds such as diolefins, which are guns precursors, and styrenic compounds (in particular styrene). Such gasolines are thus unstable which means that a hydrogenation process must be carried out before they are used. Further, depending on the conversion process, such gasolines can contain non negligible quantities of mercaptans, which must be eliminated in order to obtain xe2x80x9csweetenedxe2x80x9d gasolines, negative to the Doctor test.
Two main types of catalyst are generally used for hydrogenating diolefins and styrenic compounds: catalysts using noble group VIII metals such as palladium, and those using non noble group VIII metals such as nickel. (In the present description, the periodic table considered is that from the Chemical Abstracts Service (CAS)). However, the second type of catalyst generally has a lower activity and undersired oligomerising properties, which necessitates frequent regeneration and the use of a distillation column after hydrogenation to eliminate the heavy compounds. An example of the use of that type of catalyst is described in U.S. Pat. No. 3,691,066. Catalysts based on noble metals are generally more active than catalysts based on non noble metals.
Regarding mercaptan elimination, catalysts using non noble group VIII metals such as nickel can transform mercaptans into sulphides. However, as indicated above, that type of catalyst generally has oligomerising properties, which necessitates frequent regeneration and the use of a distillation column after hydrogenation to eliminate the heavy compounds Further, such catalysts can only treat feeds containing large quantities of marcaptans such as that found in catalytic cracking gasolines.
In addition, while catalysts based on noble group VIII metals are generally more active than catalysts based on non noble metals for hydrogenating diolefinic and styrenic compounds, they do not always transform mercaptans. Their use thus means that the hydrogenated gasoline must be sweetened, for example using a process as described in our French patent applications FR-A-2 753 717 and FR-A-2 753 718.
It should also be noted that the use of group VIII metalxe2x80x94group VIB metal systems has already been described in the literature, for example by M. Yamada, J. Yasuinaru, M. Houaila, D. Hercules in: xe2x80x9cDistribution of Molybdenum Oxidation States in Reduced Mo/Al2O3 Catalysts. Correlation with Benzene Hydrogenation Activityxe2x80x9d. J. Phys. Chem., 95, 7037-7042 (1991) or by A. Jimenez-Gonzalez, D. Schmeissr in xe2x80x9celectron Spectroscopic Studies of Mo and Pt Modified xcex3-Al2O3 Model Catalystxe2x80x9d. J. Catal., 130, 332-346 (1991). These documents, however, relate to total hydrogenation reactions.
A first aim of the present invention is to provide a novel catalyst which can carry out selective hydrogenation of diolefins and styrenic compounds in an unsaturated gasoline cut with improved performance. In this case, the catalyst of the invention has the advantage of achieving diolefins and styrenic compound conversions which are higher than that achieved with a palladium catalyst alone. The catalytic performances of this catalyst are also more stable over time than those of the monometallic system.
A further aim of the present invention is to provide a novel catalyst the use of which on an unsaturated gasoline cut containing diolefins, styrenic compounds and mercaptans enables hydrogenation of the diolefins and styrenic compounds and transformation the mercaptans in a single operation.
The catalyst of the invention is defined as comprising a particulate support constituted by grains of at least one refractory oxide, on which palladium is deposited, distributed at the periphery of the grains of the support, and at least one metal selected from molybdenum and tungsten, in the form of at least one oxide.
More particulary, the grains of the support are, for example in the form of beads or cylinders (for example cylindrical extrudes); the palladium can then be distributed in a peripheral layer of said grains with a penetration to a depth which does not exceed 80%, for example, of the radius of the beads of cylinders.
In the catalyst of the invention, the palladium content is generally in the range 0.2% to 5% by weight and the molybdenum and/or tungsten content is in the range 0.5% to 5% by weight.
The support is generally selected from refractory oxides such as alumina, silica, silica-aluiminas, magnesia or mixtures thereof. Alumina is the preferred support, mole particularly an alumina with a specific surface area in the range 5 to 200 m2/g, preferably 10 to 110 m2/g, more advantageously 20 to 80 m2/g. The pore volume of the refractory oxide support is, for example, 0.4 to 1 cm3/g.
Regarding the preparation of the catalysts of the invention, the group VIII clement and the group VIB element can be introduced using techniques which are known to the skilled person. As an example, if the group VIII metal is palladium, it can be introduced by impregnating with an aqueous or organic solution of a palladium precursor. This precursor can, for example, be an inorganic compound, for example palladium chloride or palladium nitrate, or an organometallic compound, such as palladium bis xcfx80-allyl or palladium bis-acetylacetonate. The tungsten, as an example of a group VIB element, can be introduced by impregnating the support with fin aqueous or organic solution of a tungsten precursor, for example ammonium metatungstate, ammonium paratungstate, tungsten chloride or carbonylated tungsten compounds. The molybdenum can be introduced by impregnating the support using an aqueous or organic molybdenum precursor, for example ammonium heptamolybdate.
The elements can be introduced using common or separated solutions, The two elements ale preferably introduced separately and advantageously, the compound comprising the group VIB element is introduced prior to introducing the compound comprising the group VIII element.
After introducing the different elements, the catalyst is generally dried at a temperature of 90xc2x0 C. to 150xc2x0 C., for example at about 120xc2x0 C., then calcined at temperatures of generally 150xc2x0 C. to 700xc2x0 C.
When the catalytic elements are introduced in a plurality of impregnation steps, the catalyst may undergo different treatments between two impregnation steps, for example drying at a temperature of 90xc2x0 C. to 150xc2x0 C., for example at about 120xc2x0 C., and calcining in air at a temperature of 400xc2x0 C. to 500xc2x0 C., for example at about 450xc2x0 C.
In order that the group VIB metal or metals is in the form of oxides in the catalyst, before use it generally undergoes a treatment in a reducing atmosphere, for example in a stream of hydrogen, at a temperature in the range from ambient temperature to 500xc2x0 C., more particularly in the range from ambient temperature to 250xc2x0 C.
The gasoline cut to be treated may have a high diolefin content, corresponding, for example, to a MAV (maleic unhydride value) determined using the UOP 326-82 method, in the range 20 to 100, and styrene contents of up to 5% by weight, for example, these values not being limiting. After hydrogenation, the diolefin content generally results in a MAV of less than 8, preferably less than 4 and more preferably less than 2. The styrene concentration after hydrogenation is generally less than 1% by weight, preferably 0.5% by weight.
Depending on its origin, the gasoline may contain 1 to 600 ppm of sulphur in the form of marcaptans. The process using the catalyst of the invention can transform at least 50% of the mercaptans present to sulphides. This reaction occurs during treatment aimed at hydrogenating the diolefins and/or styrenic compounds.
Treatment of the gasoline cut in the selective hydrogenation process using the catalyst of the invention comprises bringing the cut into contact with the catalyst, generally under pressure and at a temperature in the range 10xc2x0 C. to 200xc2x0 C., in the presence of a slight excess of hydrogen with respect to the stoichiometric value to enable hydrogenation of the diolefins and styrenic compounds. The hydrogen and the feed are injected as an upflow or downflow into a reactor, for example a fixed bed reactor, in which the temperature is in the range 10xc2x0 C. to 200xc2x0 C. The pressure is generally sufficient to keep more than 80% by weight, preferably more than 95% by weight of the gasoline to be treated in the liquid state at the reactor inlet. The pressure is generally in the range 4 to 50 bar, more advantageously in the range 10 to 50 bar. The hourly space velocity, HSV (defined as the ratio of the volume flow rate of the feed to the volume of catalyst) under these conditions is in the range 1 to 20hxe2x88x921, preferably in the range 2 to 10 hxe2x88x921, more preferably in tile range 3 to 10hxe2x88x921.
The catalyst of the invention can also be used in processes using different technologies. Thus, for example, the hydrogenation reaction can be carried out in a reactor within a distillation column or in a reactor which is external of and associated with a distillation column as described, for example, in French patent application FR-A-2 752 236.
The complete description of all applications, patents and publications cited above and below and the corresponding French application number 97/07 213 filed on Jun. 9th 1997 are incorporated by reference into the present description.